A wide variety of methods for chemical extraction and/or recovery are known in the art, particularly in regard to extraction and recovery of metals from materials such as mineral ores, recyclable wastes, and contaminated soils. Such methods include, for example, amalgamation that often produces health hazards and clean-up hazards; flotation that requires finely ground, de-slimed, clean, free metal; cyanide treatment that offers specific extraction, but presents environmental hazards; centrifugal concentration which works primarily for free on heavy metals; electrolytic/electrowinning processes that are expensive and slow; thermal/furnace processes that are energy intensive and expensive; and chlorination/bromination processes that require high pressure digestors that are, in turn, expensive, hazardous, and leak-prone.
Representative of the prior art, Rizet, in U.S. Pat. No. 5,549,811, teaches a process for decontamination of soils polluted with metals, wherein the polluted soil is treated with a NaOH solution to remove contaminant metals. By leaching this contaminated soil with a soda-solution containing a concentration of about 6N NaOH, the contaminants are precipitated in the form of hydroxides metals and then extracted via simple solid-liquid separation techniques. Thus, Rizet attempts to prepare contaminant metals for subsequent extraction stages well known in the art.
It will be apparent to those skilled in the art that the extraction solution taught by Rizet contains 2% hydrogen peroxide, an oxidizer, and 25-30% sodium hydroxide. His procedure includes the application of lime in a sufficient quantity to remove silica from the extract solution by effectuating precipitation of aluminosilicates that may have become solubilized. Also included is a second wash of the soil that contains the precipitated aluminosilicates with a 6N soda-solution and pH of 14, thereby tending to redissolve some of the silica that has been precipitated by the lime. By sustaining a basic medium with a pH of 14, Rizet avoids the precipitation of lead in the form of hydroxide. Besides the recovered lead, silica remains in the wash liquid.
Also indicative of the art is European Patent No. 34137 in which Reiterer discloses a process for the hydrometallurgical treatment of materials that contain zinc, wherein pulverized zinc-containing materials are initially subjected to basic leaching with alkali hydroxide. Precipitate is separate from the liquid, and the liquid is diluted with water and acidified to a pH below 7 in order to precipitate SiO. Prior to this SiO precipitation, Cl− and NH+ ions are introduced into the solution. Then, once Cu and Cd have been separated, the Zn-containing solution is subjected to an ion exchange or solvent extraction process, and, if necessary, to an electrolytic purification process.
Since Rizet discloses that approximately 1-3 g/l silica is present in the post-leaching solution, the Examiner concludes that Applicant's caustic silica solution—inherent in Applicant's chemical processing step—and concomitant treatment methodology would have been obvious to those of ordinary skill in the art.
Japanese Patent No.5000837, Laid-Open Publication No. Sho 50-8371, teaches that the removal of copper from waste liquids as a precipitate in the form of a neutral hydroxide compound. It will be appreciated by those skilled in the art that the slimy nature of this precipitate renders it difficult to settle, and, of course, such adverse settling characteristics are known to be improved by incorporating silica powder into the alkaline solution. The amount of caustic required prerequisite for precipitation of small quantities of copper, such as 400 ppm copper in stoichiometric amounts, e.g., approximately 220 ppm hydroxide ion or 500 ppm caustic is obviously still small. As clearly demonstrated in this Japanese Patent, excess caustic usage is not necessary.
Each of these approaches use an initial alkali-based leaching step to attempt to solubilize the metal contaminant portion and the like so that it may be extracted from the contaminated material. Unfortunately, as will be appreciated by those skilled in the art, only limited extraction of metals from ores, dust, and the like has been hereinbefore obtained using conventional separation methods. It would be advantageous for promoting extraction of metals from such contaminated materials if a leaching solution and concomitant methodology were developed that is capable of achieving levels of metal extraction heretofore unknown in the art.
The present invention overcomes many of the disadvantages of known extraction and recovery methods by providing liquor compositions that, while leaching the contaminated materials under mild conditions, effect a chemical change in the underlying structure of the contaminated materials wherein a permanent proclivity for metal extraction and recovery is attained. The present invention performs this extraction and recovery function while generating relatively innocuous by-products or wastes.